(1) Field of the Invention
The present invention relates to the field of the penetration of material, such as bone structures or the like, that exhibit differences in intrinsic physical quantities.
The present invention relates more particularly to a device for monitoring the penetration of a penetrating means into anatomical structures, and especially bone structures, of a living body, said structures having at least two regions of different electrical impedance.
(2) Prior Art
In spinal surgery, for example, when drilling the pedicle, it frequently happens that the vertebral bone cortex is pierced, broken or chipped by the penetrating drill bit. Depending on the authors, 15 to 40% of pedicle screws are thus improperly positioned, with a clinical manifestation (pain, paralysis, hemorrhaging, etc.) in 1 to 2% of the cases and therefore require further intervention.
Surgeons sometimes use equipment such as the following:                equipment for surgical navigation, which is expensive and cumbersome to implement;        equipment for monitoring of sensory and/or motor evoked potentials, this being less expensive but also restricting as it requires the presence of a specialist whose mission is solely to carry out this monitoring operation.        
As a result, in most cases the operators rely solely on their knowledge of anatomy and their experience in order to accomplish this risky surgical procedure.
The foregoing is also true in the case of other surgical fields.
In the field of drilling bone bodies, the prior art also includes international patent application WO 01/01875.
This patent application discloses a device that uses the ability of nerves and muscles to transmit signals, in order to warn the user when the drilling means comes into contact with a nerve, so as to prevent any lesion of said nerve.
The prior art also teaches the use of impedance measurement in medical devices.
For example, the prior art includes the United States Patent U.S. Pat. No. 4,630,615 which relates to a neural stimulation system that incorporates an apparatus for measuring or determining impedance in which it is desired to manage and determine the changes in impedance in a guide connected to a cathode implanted into the epidural space of a spinal column. Such a neural stimulation system is typically used when it is desired to block off pain signals that are sent to the brain, which system may also be used for the treatment and/or relief of symptoms involving uncoordinated movements of the spinal column, such as epilepsy, spasticity, cerebral paralysis, etc.
The prior art also includes British Patent Application GB 2 335 990 that relates to a system for mechanical penetration of a needle and for stopping the penetration if a different impedance is detected at the end of the needle.
The object of that system is to inform the user when the end of the needle has definitely reached the desired depth, either using an impedance value or using a change in impedance.
The object of that system is thus absolutely not to prevent a determined depth being reached.
Furthermore, that system requires, for each patient, an experimental determination of the impedance values or of the changes in impedance, as explained on the last page of that application, which absolutely does not allow this system to be adapted for the purpose of preventing a determined depth being reached.
The prior art also includes French Patent Application FR 2 101 911 which relates to an instrument for measuring the length of the root canal of a tooth.
That instrument is used to deduce the length of a hollow body by the subsequent measurement of the distance between two reference marks after these two reference marks have been placed into longitudinal abutment, respectively in the bottom of the tooth canal in the case of the probe, and the crown or the upper edge of the cavity in the case of the rubber piece.
That instrument allows the user to be informed, after having made a cavity, about the depth of this cavity; it does not allow real-time monitoring of the penetration of a penetrating means into anatomical structures.
Another drawback of the techniques of the prior art is that are cumbersome to implement and tend to extend the operation time, thus increasing the associated risks.